Group III nitride semiconductors are suitable for making high-temperature, high conversion speed and high power electronic devices due to properties such as wide band gap, high dielectric breakdown electric field and high electron saturated drift velocity. In a GaN-based field effect transistor, a large amount of charges are generated in a channel layer by piezoelectric polarization and spontaneous polarization. Since two-dimensional electron gas is formed by ionization of donor-type surface states of a surface of nitride, a current density of a nitride transistor is sensitive to the surface states, and a current collapse effect is prone to occur. In addition, during epitaxial growth of a GaN-based electronic device, there are many defects on a material surface because of lattice mismatch and thus tensile stress introduced into AlGaN by a GaN buffer layer. These surface defects may also affect on performance of the device, such as causing a current collapse effect and causing problems in reliability of the device. GaN-based field effect transistors may be divided into two categories, i.e., Schottky gate field effect transistors and insulating gate field effect transistors, from a view of gate structure. A gate with a Schottky contact is easy to be fabricated and has an easily-controlled surface, which are ideal for a radio frequency device. However, the gate with the Schottky contact has no dielectric layer for separation, and therefore a leakage current of the gate is high. In addition, due to limitation of forward conduction of the Schottky contact, a bias voltage on the gate should not be greater than 2V, and a bias voltage greater than 2V will cause the gate to lose an ability of controlling the channel. For an insulating gate, a dielectric layer, such as silicon dioxide, aluminum oxide, hafnium oxide, silicon nitride and silicon oxynitride, is usually arranged below gate metal, and thus the leakage current of the gate is low, which is suitable for power devices.
There are generally three structures for conventional nitride transistors, as shown in FIGS. 1a, 1b and 1c. In fabrication of a gallium nitride transistor as shown in FIG. 1a, a surface of nitride is exposed by etching a dielectric layer 6 in a gate region, to form a gate 11 with a Schottky contact. A disadvantage of this method is that the surface of the nitride is prone to be damaged during dry etching, which increases the surface states and thus degrades performance of the device. In addition, the Schottky contact brings a high reverse leakage current, which causes problems in reliability of the device. In order to reduce the leakage current of the gate, an insulating layer may be deposited at the gate region after etching away a silicon nitride passivation layer in the gate region, the insulating dielectric layer may be deposited with a device such as ALD for example, as shown in FIG. 1b. However, during the etching and the deposition of the dielectric layer 6, the surface of the nitride transistor 1 will be damaged, in addition, the surface of the nitride transistor 1 will be contaminated to some extent before depositing the ALD dielectric layer. All these factors will increase the surface states and degrade performance of the device. Interface state between the ALD dielectric layer and the nitride is also a major unsolved problem which may cause a severe current collapse effect. Another structure can also be used to achieve an insulating gate nitride field effect transistor, as shown in FIG. 1c. In etching the dielectric layer at the gate region, a part of the dielectric layer 6 under the gate may be retained by controlling etching speed and etching time. However, due to repetition of the etching process, a thickness of the retained dielectric layer can not be accurately controlled, thereby causing a drift of a threshold voltage.